Provided that the shape, size, and composition profile of semiconductor-embedded quantum dots are given, theory is able to accurately calculate the excitonic transitions, including the effects of inhomogeneous strain, alloy fluctuations, electron-hole binding, and multiband and intervalley coupling. While experiment can accurately provide the spectroscopic signature of the excitonic transitions, accurate determination of the size, shape, and composition profile of such dots is still difficult. We show how one can arrive at a consistent picture of both the material and the electronic structure by interactive iteration between theory and experiment. Using high-resolution transmission electron microscopy, electron-energy-loss spectroscopy, and photoluminescence (PL)more » spectroscopy in conjunction with atomistic empirical pseudopotential calculations, we establish a model consistent with both the observed material structure and measured electronic/optical properties of a quantum dot sample. The structural model with best agreement between measured and predicted PL is a truncated cone with height 35 {angstrom}, base diameter 200 {angstrom}, and top diameter 160 {angstrom}, having a nonuniform, peaked composition profile with average 60% In content. Next, we use our best structure to study the effect of varying (i) the amount of In in the dots, and (ii) the spatial distribution of In within the dots. We find that by either increasing the amount of In within the dot or by concentrating a given amount of In near the center of the dot, both electrons and holes become more strongly bound to the dot. A small change of In content from 50 to 60% causes an exciton redshift of about 70 meV. Changing the composition profile from a uniform In distribution to a centrally peaked distribution can redshift the exciton by an additional 20--40 meV.« less

A formation process for long chains of quantum dots during the molecular-beam epitaxial growth of (In,Ga)As/GaAs(100) multilayers is presented. The morphology evolution monitored by atomic force microscopy for a series of (In,Ga)As layers demonstrates that the highly anisotropic lateral alignment of dots is gradually developed as the result of the strain field interaction mediated by the GaAs spacer coupled with the anisotropic surface kinetics that occurs during capping the dots. The dot-chain structure, providing unique properties of its own, is demonstrated to serve as a template for the spatially controlled growth of strained quantum dots in general.

In{sub x}Ga{sub 1-x}As/GaAs quantum dots (QDs) were grown by solid source molecular beam epitaxy for indium contents of around 30%, which assures the QD growth in the very low strain limit. The structures were fabricated for a constant nominal In{sub x}Ga{sub 1-x}As layer thickness but varying content (strain) from below to far above the critical thickness conditions, which has allowed to detect the onset of three-dimensional island formation and their evolution with the increasing material amount (for higher In contents the critical thickness for island formation is smaller and hence a larger fraction of the In{sub x}Ga{sub 1-x}As layer ismore » spent on dot formation). In order to investigate the properties of such an uncommon QD system, photoreflectance and photoluminescence, combined with scanning electron microscopy, have been used. Optical transitions connected with the ternary layer have been observed and followed from the lowest content quantum well case through the transformation into three-dimensional islands on the wetting layer (WL) and a coexistence of the QD-related and WL-related transitions. Due to the observation of both heavy hole and light hole related transitions in photoreflectance spectra, the thickness of the wetting layer versus changed indium content could be determined, comparing the experimental data with the results of the effective mass envelope function calculations.« less

We have applied ion channeling techniques to investigate effects of proton irradiation combined with thermal annealing on In-Ga atomic intermixing in a self-assembled InAs/GaAs quantum dot (QD) system. A molecular-beam-epitaxy grown InAs/GaAs QD sample was first irradiated with 1.0 MeV protons to a dose of 10{sup 14} cm{sup -2} and subsequently annealed at temperatures between 300-700 deg. C. Ion channeling measurements indicate that such postgrowth processing leads to an enhanced amount of In atoms registering along the <100> growth direction. This observation yields direct evidence for the occurrence of In-Ga atomic intermixing in the QD structure during postirradiation annealing. Themore » effective activation energy for such intermixing process is determined to be {approx}0.2 eV. Furthermore, ion channeling data suggest three distinct stages for In-Ga atomic intermixing processes in the QD system, with In-Ga interdiffusion proceeding vertically along the growth direction or laterally in the QD layer, depending on postirradiation annealing temperatures.« less

We show how an atomistic pseudopotential plus many-body configuration interaction theory can address the main spectroscopic features of self-assembled dots including, excitons, trions, biexcitons, fine-structure, charging spectra as well as electric-field dependence of entanglement in dot molecules.